This invention relates to the chemical modification of the surfaces of inorganic solids with organic molecules. Such chemically modified, inorganic solids serve, for example as stationary phases in chromatography, in catalysts, etc.
The use of solids as stationary phases in chromatography and catalysis is conventional. Such stationary phases have an activity which is too high and are unsuitable for many chromatographic separation problems. By chemical modification of the surface of inorganic solids and by the introduction of the desired functional groups, the physical and chemical properties of the solids can be influenced.
Several attempts have been made to alter the surface of inorganic solids (e.g., of silicon dioxide). Three basic methods can be differentiated, i.e., physically coating the solid body with liquid or solid substances; polymerization of organic reagents onto the surface; and chemical reaction of the --OH groups present on the surface of the solid body.
In chromatographic separation processes, the sample substances present in the gas or liquid mobile phase interact with the stationary phase, and separation takes place by sorption. Solids coated with liquid or solid substances have the disadvantage that the stationary phase "bleeds," i.e., the mobile phase becomes saturated with the stationary phase and is carried out of the separation column. This is especially prevalent in liquid chromatography, and accordingly the use of such stationary phases is greatly limited.
Stationary phases produced by the polymerization of organosilicon compounds, e.g., see C. R. Hastings, W. A. Aue, J. M. Augl, J. Chromatog., 53: 487-506 (1970), show low mass transfer velocities because the resultant layer thickness is too large. Another disadvantage of such stationary phases is that they do not contain any functional groups. However, the selectivity of the stationary phases depends on the respectively present functional groups.
The third method for modifying solid body surfaces is to chemically react the --OH groups present on the solid surface. The oldest such method is the silanization of solid bodies containing silicon dioxide with chloroalkyl silanes. The resultant weakly acidic --Si--OH groups can then be esterified with alcohols; by the use of substituted alcohols, several desired functional groups can also be introduced, e.g., as described in German Pat. No. 1,902,226. While generally satisfactory, the solid bodies modified in this way are susceptable to hydrolysis and thermal degradation.
Another process for the modification of silicon dioxide is disclosed in German patent application No. P 22 36 862.3-41 published Jan. 31, 1974. According to this process silanol groups, e.g., of SiO.sub.2, are converted into --Si--Cl groups by chlorination with thionyl chloride. The chlorinated silicon dioxide (e.g., silica gel) is subsequently reacted with amines to form --Si--N bonds. Such products are hydrolysis-resistant but only in a fairly narrow pH range of 4-8, which is insufficient for many applications. For example, in case of such phases, it is impossible to safely regenerate the HSO.sub.3 groups (--NaSO.sub.3 + HCl .fwdarw. HSO.sub.3 + NaCl), which occupy a special position in cation exchange (e.g., demineralization of water and liquid chromatography) and in catalytic processes; dilute acids, e.g., 1--2N HCl, are employed for the regeneration, which have a pH of below 1. Under these conditions, the --Si--N bond is quickly split and the organic portion detached from the solid body.
U.S. Pat. No. 3,664,967 describes the silanization of silica gel and alumina with alkyl chlorosilanes (having a chain length of C.sub.12 - C.sub.28) and vinyl halosilanes. According to this patent, the solids are reacted with alkyl and vinyl chlorosilanes at room temperatures for 3-6 hours and then the residual --Si--Cl groups are reacted with alcohols of a chain length of C.sub.6 - C.sub.28 with the formation of Si--O--C-- linkages. Following halogenation, such phases are utilized in liquid chromatography. A disadvantage of these stationary phases is that they contain Si O--C as well as Si--C bonds. It is known that the Si--O--C bond is sensitive to hydrolysis, and accordingly aqueous fluid phases cannot be used with such stationary phases. Furthermore, since these stationary phases do not contain any functional groups, it is impossible to vary their selectivity.